Aid worker Olivier Vandecasteele, 42, was arrested on a visit to Iran in February 2022 and sentenced in January to 40 years in prison and 74 lashes on charges including spying.

The Belgian government said on Friday they had reason to believe that Iran had been planning to increase his sentence.

Iranian diplomat Asadollah Assadi was convicted in Belgium in 2021 in connection with a foiled bomb plot in France and sentenced to 20 years in prison.

Both countries rejected the charges against their citizens as fabricated.

"As I speak, Belgium's Olivier Vandecasteele is on his way to Belgium. If all goes to plan, he'll be with us this evening. Free at last," Belgian Prime Minister Alexander De Croo said on Friday.

An official with the Belgian government said that Vandecasteele's condition was "relatively okay for someone who has been isolated for that long".

Iranian state TV reported that Assadi had arrived back in Tehran and showed him sitting with officials including government spokesman Ali Bahadori Jahromi.

Iranian Foreign Minister Hossein Amirabdollahian described Assadi in a Twitter post as "our country's innocent diplomat who was illegally arrested against international law".

Belgium's constitutional court in March upheld a prisoner exchange treaty with Iran but that was not what was used to free Vandecasteele.

Using the treaty would have taken too long, Belgian government officials said, as that would have meant that the target of the bomb plot in France, the exiled National Council of Resistance of Iran (NCRI), could have gone to court to stop the transfer.

Instead, Belgium used Article 167 of its constitution which specifies that the government can conduct its own foreign policy and regulate international relations, a government official said.

NCRI said Assadi's release 15 years before the end of his sentence encouraged terrorism.

"Nothing justifies releasing a terror master," Shahin Gobadi from NCRI said.

Belgian opposition party N-VA said in a statement the government had succumbed to Iran's blackmail politics.

Oman's foreign ministry said earlier an agreement had been reached under which the prisoners were released and transported from Brussels and Tehran to Muscat, the Omani capital, on Friday in preparation for their repatriation.

The Gulf Arab country has good relations with both Iran and European countries and has acted before as a mediator for the two estranged sides.

Oman's Sultan Haitham bin Tariq al-Said is scheduled to travel to Iran on Sunday for a two-day visit.

Australian Associated Press